1. Field of the Invention
The present invention concerns an illumination system for wavelengths of ≦193 nm, wherein the illumination system includes an object plane and a plane conjugated to the object plane, and also includes a collector unit having a mirror shell. A beam having a bundle of rays is emitted from a light source situated in the object plane. The rays impinge on the mirror shell at an angle of incidence of <20° relative to a surface tangent of the mirror shell. This configuration minimizes degradation of the optical components of the illumination system that could be caused by debris produced by the light source.
2. Description of the Prior Art
In order to further reduce pattern widths for electronic components, particularly to the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography. The use of light with wavelengths smaller than 193 nm is conceivable, for example, by lithography with soft x-rays, so-called EUV lithography.
EUV lithography is one of the most promising future lithography techniques. At present, wavelengths in the range of 11-14 nm, especially 13.5 nm, are being discussed as wavelengths for EUV lithography, with a numerical aperture of 0.2-0.3. Image quality in EUV lithography is determined, on the one hand, by a projection objective, and, on the other hand, by an illumination system. The illumination system should provide the most uniform possible illumination of a field plane, in which a pattern-bearing mask, a so-called reticle, is arranged. The projection objective images an object situated in the field plane into an image plane, a so-called wafer plane, in which a light-sensitive object is arranged. Projection exposure systems for EUV lithography are designed with reflective optical elements. The form of the field of an EUV projection exposure system is typically that of an annular or ring field. Projection systems usually operate in scanning mode. An EUV projection exposure system has been made known, for example, from EP-A-1,026,547 or EP-A-0 939,341.
Collectors in refractive illumination systems are well known, for example see Lexikon der Optik, Leipzig 1990, edited by Heinz Haferkorn, page 183. Collectors in EUV-systems function in a manner similar to that of collectors in refractive systems.
Both EP-A-1,026,547 and EP-A-0 939,341 show illumination systems, in which the light of a light source is taken up by a first collector and is imaged into an intermediate image in an intermediate image plane. It has become known from EP-A-0 939,341 to arrange a second collector in the light path from the light source to the field plane after the intermediate image plane. The collectors shown in both EP-A 1,026,547 and EP-A-0 939,341 are normal incidence mirrors with multilayer coatings.
If, for example, light sources such as those described in U.S. Pat. No. 5,763,930 are used as light sources for EUV lithography, then the problem arises that debris, for example, abraded material or dirt particles, contaminate surfaces of the normal-incidence mirrors after a short time and thus reduce the reflectivity of these components.
Nested collectors for wavelengths of ≦193 nm, particularly wavelengths in the range of x-ray radiation, have become known from a multiple number of documents.
Thus, U.S. Pat. No. 5,768,339 shows a collimator for x-ray radiation, wherein the collimator has several nested paraboloid-shaped reflectors. The collimator according to U.S. Pat. No. 5,768,339 serves for shaping an isotropically emitted beam bundle of an x-ray light source into a parallel beam.
A nested collector for x-ray radiation that serves for collimating isotropic x-ray radiation emitted from a source into a parallel beam bundle, as in the case of U.S. Pat. No. 5,768,339, has become known from U.S. Pat. No. 1,865,441.
U.S. Pat. No. 5,745,547 shows several arrangements of multi-channel optics, which serve for focusing radiation of a source, particularly x-ray radiation, via multiple reflections, at one point. In order to achieve a particularly high transmission efficiency, U.S. Pat. No. 5,745,547 proposes elliptically shaped reflectors.
An arrangement has become known from DE 3,001,059 C2 for use in x-ray radiation lithography systems, which has parabolic nested mirrors arranged between an x-ray radiation source and a mask. These mirrors are arranged in such a way that divergent x-rays are shaped into a parallel-running output beam bundle.
An arrangement of nested reflectors that has become known from WO 99/27542 is used in an x-ray proximity lithography system for refocusing light of a light source so that a secondary light source is formed. The nested reflectors may have an ellipsoid form.
A nested reflector for high-energy photon sources has become known from U.S. Pat. No. 6,064,072. The nested reflector serves for shaping divergent x-rays into a parallel-running beam bundle.
WO 00/63922 shows a nested collector that serves for collimating a neutron beam.
A nested collector for x-rays has become known from WO 01/08162, which is characterized by a surface roughness of less than 12 Å rms of the inner, reflecting surfaces of individual mirror dishes. The collectors shown in WO 01/08162 also comprise systems with multiple reflections, particularly Wolter systems, and are characterized by a high resolution, as is required, for example, for x-ray lithography.